With a little help... KT88 PP UL Amp

[ray_moth]

You may notice that there is a step-network in the circuit in Hafler's article. This will deal handily with the problem of LF instability. However, it does complicate the biasing requirements.

The circuit by Franz Wichlas is deficient, IMHO, because of limited headroom of the concertina splitter. There needs to be intervening amplification, as in the Williamson. The only OP tubes that can reasonably be driven directly from a concertina splitter are low-power pentodes (e.g. EL84, 6V6).

[Herr Grau]

Originally Posted by richwalters

The Plitron is a gapless core: the Sowter is conventional E&I. Swapping to the Plitron in a Williamson can be done...

Thanks for the answer. This far I can follow...

Originally Posted by richwalters

...but one effectively alters the Bode map so the loop gains when global nfb is used have to be re-worked to avoid LF instability. Reminds me of the Eckland amp:may be worth examination.

(Although the discussed schematic ist from John Eckland...)

...and here we're leaving my territory.
Can this re-work be done from afar? (w/o the circuit)
Cause I somewhat get the funny feelings actual problems cause...
Ordering Plitrons is out of question. They would charge far over 100$ just for shipping. I'm still a student after all.

Originally Posted by 88man

Eli D. pointed out that the Williamson circuit requires top notch transformers to prevent unstable phase shift oscillation. He says that the Mullard circuit is more stable in this regard.

I had read the thread an didn't worry too much about that, since I understood Sowter made some of the best OPTs available. Dont they?

Originally Posted by SY

"[...]Separation of the time constants permits less phase shift for the same frequency loss. Increasing one pair of coupling capacitors from .05uF to .25uF gives a five-to-one ratio of time constant for the two pairs of networks and increases the low frequency stability margin at nominal increase in cost."

In this schematic the coupling capacitors are the 0,22uF between the tubes, right? How would you modify them? Decrease the one pair or increase the other one?

[SY]

He's done that for you by staggering the R part of the time constant. There's a 5:1 spread in this design, just like in Hafler's modification. 10:1 would be even better, but probably not necessary. The Eckland circuit is an almost-exact copy of Hafler, with a standard KT-88 UL output stage. No trick circuitry, just standard, well-established stuff. Which is a good thing, IMO.

[Herr Grau]

Thanks SY, one thing less to worry about.

[Herr Grau]

Leaves the question of the OPT.
I'd realy like to use Sowter, but I also have access to Rondo-Müller and Amplimo to get toroidal OPTs. Would that make things easier?

[jwjensen356]

I don't think you can go wrong with this one:
http://vacuumtubebrasil.profusehost....0operation.pdf

This was after Herbert Keroes and David Haller parted company, Hafler founding Dynaco and Keroes Acro Products. This amplifier uses the then-new 6550 and the Acro 330 OPT. Note that it has a vacuum tube rectifier.

I built two of these 50 years ago (!!!) and they are still very solid. I've variously used 6550s and KT-88s. Right now there are pairs of the Russian Gold Lion KT-88s in place. I used WWII surplus oil capacitors for the 500V KT-88 plates and electrolytics for the 6SN7 circuitry. Construction was with military-style terminal boards. I substituted the Dynaco 430 for the Acro 330 (The 430 was $30 then, the 330 was $40, big savings for a student at the time), The power transformers are surplused replacement transformers for the RCA 630 TV chassis. Surplused HV capacitors and power transformers were purchased at the original Radio Shack store in Boston (I'm estimating total cost for all to be $10 maximum)

Reliability through the years has been very good with minimal problems. Output tube replacement of course. Several years ago the electrolyics were replaced. Also, the inter-stage coupling capacitors were replaced (also stabilizing the output tube bias setting).

I'll never replace them.

John

Originally Posted by Herr Grau

Which design would you recommend?
I'm searching for the definite amp for the next many years, so I'm more than willing to consider every option, of course searching for the best value. I just need a complete, working schematic which I can 'cook' the amp from.

If you don't think the design is 'the thing', than the PSU-work isn't necessary now.

I don't have too many wishes, I want a KT88 PP Amp with non-rare pre-tubes (personaly I do like the 6SN7, but thats not so important)(yeah, it shouldn't need an external pre-amp...) and tube rectification. It would be most terrific if the design would work with one of the standard supplied Sowter OPTs...

[Herr Grau]

It looks good to my amateur eyes. Although I don't get the thing with all the potis and it's not good that it uses a third transformer. Every transformer is a major cost factor. Whats with the taps marked "x" and "y"?

Any other oppinions to which design should be preferred?

[SY]

T3 just generates the bias voltage by upconverting 6.3V to 120V. It's very low current, so the transformer costs maybe $4 or so. Not a major deal. It can also be done with a separate winding or a tap on the main power transformer.

For European voltages, you'd want a 220/12.6V to get the same output.

[jwjensen356]

Originally Posted by Herr Grau

It looks good to my amateur eyes. Although I don't get the thing with all the potis and it's not good that it uses a third transformer. Every transformer is a major cost factor. Whats with the taps marked "x" and "y"?

Any other oppinions to which design should be preferred?

In those days (the 50s) preamps were often powered by B+ available from the power amplifier (see octal socket on chassis skirt in the photo). The last few sentences in the section 'Amplifer Circuit' addresses this and the 'x' and 'y'. Eliminating this simplifies the circuita and eliminates one pot. The other pot provides for balancing of the output tubes.

John

[Herr Grau]

You're right, if I had read more carefuly...

Can I simply substitute the Selenium Rectifier with any other 50mA Si or Ge Rectifier?

Aaaand of course: Are the changes I made correct? (The removal of the pre-power-part...)
He says in the article that you should ground the middle tap of the 6,3V-winding if you remove the pre-power-part; should I order a transformer with middle tap in this winding or just ignore this information?